This invention relates to fluid flow control systems, in particular to fuel delivery systems for gas turbine engines.
Fuel delivery systems for gas turbine engines typically fall into one of two classes, based on whether the source of pressurized fuel is a positive displacement pump or a non-positive displacement source, such as a centrifugal or vortex pump. Positive displacement pump systems are typically very complex and expensive as compared with non-positive displacement systems, which have a significant advantage in terms of the cost and simplicity of the metering system. Since the source of pressurized fuel in a non-positive displacement system is at a constant pressure irrespective of flow rate, the flow metering valve can be a simple metered orifice, such as the fuel metering valve disclosed in U.S. Pat. No. 5,368,273, which is incorporated herein by reference.
A conventional non-positive displacement fuel system capable of open loop operation utilizes a "delta-p" regulator, which is a special pressure regulator adapted to provide a fixed pressure drop across the metering valve irrespective of the fuel flow rate. It is well known that the flow rate of a substantially incompressible fluid such as jet fuel across a fixed pressure drop is proportional to the size of the metering orifice. Accordingly, by fixing the pressure drop across a metering valve, fuel flow through the metering valve is accurately controlled simply by adjusting the size of the metering orifice, for example by moving a needle valve in and out of the orifice.
Certain metering valves used in non-positive displacement fuel systems are limited, however, in their ability to meet the full range of flow requirements of modern high performance turbine engines. If operated at a first pressure differential, although the minimum flow requirements can be met, the maximum flow requirements cannot be met due to head losses in the system. If operated at a second, higher, pressure differential, the maximum flow requirements can be met, but the minimum flow requirements cannot be met within an acceptable degree of linearity due to the dominance of the non-linear effects of a severely choked metering orifice.